The present invention relates to apparatus for gamma correction of signals generated by a television film camera, and in particular to negative gamma correction of negative film used with a telecine camera.
In a television system, it is necessary to process the video signals before transmission to compensate for certain nonlinearities in the respective transmission and receiving systems to ensure that the viewer sees a picture which is a true reproduction of the televised scene. Among the nonlinearities of the system for which compensation must be made are the gamma characteristics of the television receiver picture tube and the televison camera pickup tube.
Gamma is defined as a numerical indication of the degree of contrast in a television image. Kinescopes used in television receivers generally have a nonlinear characteristic such that the black portions of a video signal are compressed and the white portions of a video signal are stretched. The black to white range, or gray scale, of a monochrome television signal or the luminance portion of a color television signal is represented by amplitude variations of the video signals. Therefore, a video signal varying linearly in amplitude applied to a nonlinear kinescope in a television receiver would result in a picture the contrast range of which would be reduced undesirably according to the nonlinear transfer characteristic of the kinescope. Accordingly, it is desirable to gamma correct the video signal prior to transmission in such a manner that the signal reproduced in a television receiver has the desired contrast range.
Generally, gamma correction is accomplished by passing the video signals derived from the television camera through a nonlinear circuit having a predetermined exponential relationship between input and output to precorrect the signal for the subsequent nonlinear transfer characteristic of the kinescope in the television receiver. While the exponent may be any selected number, it is generally accepted that the nonlinear circuit should provide an output equal to its input raised to reciprocal of the kinescope gamma, typically to the one-half power. The nonlinear circuit is usually located in a video signal processing amplifier coupled between the camera pickup tube and the color encoder. When the image source for the television camera is a positive film such as generally used for television programming, the positive film has a gamma characteristic which may be expressed in video signal form as V.sub.X = K.B.sup..gamma., where K is a constant and B is the scene brightness. For a typical positive color film, gamma will be approximately +2. This gamma value of +2 for a positive film when combined with the signal processing gamma of +0.5, previously described, results in an overall gamma of approximately + 1. This combined signal gamma value of +1 is further combined at the television receiver kinescope whose gamma is typically +2. The overall combined gamma from film to signal processing to kinescope yields a gamma value of +1 which is desired in order to accurately reproduce the film images.
Heretofore, it has been customary practice in television programming to utilize positive color film in television film cameras because such positive film is color balanced; that is, equal amounts of red, green and blue colors yield white, thereby permitting direct viewing of the positive film by the camera system. However, the making of positive film from the original negative film requres at least one extra processing step which takes time and results in a degradation or resolution as well as color saturation of the resulting positive film relative to the negative. It would be advantageous to use negative film directly. However, negative film is not color balanced; therefore, a masking operation is utilized for balancing the video output signal from negative film to correspond to the balanced video signals from a positive film. Apparatus suitable for this masking operation is described in detail in co-pending application of Harold G. Seer, Jr. Ser. No. 661,615, assigned to the same assignee and filed concurrently herewith entitled, "Negative Color Mask Correction".
negative masking operations will provide for the development of balanced video signals when utilizing negative film; however, such masking operations do not correct for the negative gamma characteristic of negative film. As was previously described in conjunction with the transfer characteristic of television systems, known positive gamma circuits readily accommodate a positive film, whereas negative gamma image signal sources cannot be readily accommodated without additional circuitry. Prior art efforts involved inverting the video signal and providing negative curve shaping by means of networks utilizing semiconductors and resistive components in an attempt to develop a video signal with a positive gamma from negative film sources. These typical prior art attempts were only marginally successful in regard to satisfactory curve shaping of the inverted video signal as well as tracking repeatability between the three color channels.